The invention relates to a method of machining by erosive electrical discharges initiated and supplied with current by a succession of voltage pulses applied between an electrode tool and an electrode workpiece, according to which a relative displacement between these electrodes is achieved along a cyclic translation movement following a closed path inscribed in a surface of predetermined shape and in such a manner as to decrease the gap between these electrodes on a portion of the machined surface and to produce a cyclic displacement of the machining zone.
Machining by electro-erosion comprises in general a phase of rough shaping followed by one or several finishing phases during which the machining rate and the gap comprised between the electrode tool and the electrode workpiece are progressively and simultaneously decreased. A known machining process (French Pat. No. 1,274,953) permits to effectuate all these operations with the same electrode tool. The process consists in displacing the electrodes one relative to the other according to a loop shaped path such as to reduce the machining gap to the value corresponding to the rate of machining imposed on a portion of the surface to be machined and to provide a cyclic displacement of the machining zone on that surface. This displacement is a translation which may be executed according to a circular path or according to a rectangular path if the surfaces to machine are orthogonal, or again according to a path inscribed in a surface of predetermined form or shape.
This known machining process presents a drawback when machining surfaces having a section of variable curvature or when machining plane surfaces of different sizes. For example, in the case of machining a surface of variable curvature by effecting a circular translation movement, the machining zone is displaced at a speed varying proportionally to the radius of curvature of this surface. As the removal of material is inversely proportional to the speed of displacement of the machining zone, the surface portions of small curvature radius are machined more rapidly than the surface portions of large curvature radius, so that the machining gap varies during each translation cycle and causes variations in the machining current, resulting consequently in a decrease of the efficiency of machining. The machining gap becomes progressively wider and, as a result, machining of the surfaces of small radius of curvature stops and an important loss of machining efficiency occurs at each cycle of the translation movement.
The same phenomenon arises in the case of plane orthogonal surfaces of different sizes, for example, when machining lateral surfaces of a rectangular cavity. In this case, owing to the fact that each surface is subjected to electrical discharges at the same time, the density of discharges is larger on the surface of small size than on those of large size, such that the machining progresses more rapidly on the surfaces of small size and the machining current decreases during machining of these surfaces until it becomes nil. In addition, the increase of the gap at the angles prevents the electrical discharges from occuring during a relatively long time and results in an important drop in efficiency.